In paper making there is generally a need to increase the opacity and brightness of paper by the use of fillers, for instance titanium dioxide which is incorporated into the fibrous paper web by wet-end addition into the paper making process. Titanium dioxide is a white pigment well known as a filler additive. It is, however, an expensive additive. A reduction of the amount of titanium dioxide required to achieve the standard for a given paper grade utilizing this filler would be a tremendous cost savings. For some paper grades, for instance highly filled light weight paper, there is a great demand for higher levels of brightness and opacity but the amounts of titanium dioxide now necessary for such levels are cost prohibitive.
The cost problems attendant on the use of titanium dioxide filler are severely aggravated by the standard method of retention of the filler on the paper fibers. This standard method is by way of agglomeration of the primary or ultimate titanium dioxide particles into aggregates of particles sufficiently large to be caught and retained within the fibrous web of paper. In this agglomeration method, retention aids are used to flocculate the titanium dioxide particles. Such retention aids are generally polymeric materials believed to act by bridging the discrete particles, retention aid molecules adhering to a plurality of titanium dioxide particles by adsorption. Aggregates sufficiently large for retention by such standard filtration methods, however, are far from optimum particle size for the opacifying function of the filler. A filler's opacifying ability is a function of its ability to scatter light, i.e., return incident light by reflection. Scattering depends on the refractive index of the filler, and increases with the magnitude of the difference in the refractive index between filler and surrounding medium. For a given filler its refractive index is a constant. The light scattering for a given amount of filler, however, depends also on the surface area of the filler available for such function. Aggregation or agglomeration of many discrete particles into fewer and larger units greatly diminishes the surface area of the filler available for light scattering. Hence the opacifying efficiency of a filler such as titanium dioxide would be increased if its particles were present in use environment as well dispersed and distinct, or small clusters of particles approaching the original primary particle size. Increasing the opacifying efficiency would permit reduction of the titanium dioxide used to provide opacity of the standard desired.
In addition, with the standard agglomeration method some of the titanium dioxide will not be retained on the paper web, wasting this expensive filler and at times loading the white water system. Hence increasing retention of the titanium dioxide on the paper web will reduce the amount required for a given standard.
Another deficiency of the standard method is the energy consumption, and its concommitant costs, required to keep titanium dioxide suspensions from settling during processing. Intense mechanical agitation is required to overcome titanium dioxide's fast settling rate. It would be a cost savings to provide more stable titanium dioxide dispersions.
The agglomeration and filtration method can also lead to "two-sidedness", an undesirable condition where there is a higher concentration of filler on the felt side of the paper than on the wire side. The two sides will then differ as to brightness, print quality, pick resistance and other properties. Hence it is highly desirable to provide a method of titanium dioxide retention that more evenly distributes this pigment throughout the paper web, avoiding two-sidedness and paper curling.
Given these problems, there is a serious need for a means to stabilize titanium dioxide suspensions used in wet-end application to paper and a means to promote the attachment of titanium dioxide as discrete pigment particles or small clusters of particles to paper fibers evenly without interference with the interfiber bonding. A combination of higher retention and increase of optical properties would reduce the costs of using titanium dioxide for all paper grades. For the now expanding lightweight, highly filled paper industry, such combination would provide significantly increased brightness and opacity with small filler additions. Further, stronger titanium dioxide attachment to paper fibers may decrease on-machine dusting problems sometimes encountered.
Titanium dioxide is also known to auto-flocculate, i.e., convert from discrete particles to larger aggregates, when introduced into hard water streams present in many paper machine systems. A high retention, low particle size, method and means must also inhibit such auto-flocculation mechanism to be widely commercially acceptable.